In an image forming apparatus based on electrophotography, such as a printer or a copier, a toner image corresponding to image data is transferred to a recording sheet, such as plain paper or an OHP sheet, and is then fixed by a fixing device. The fixing device heats and applies pressure to the toner image on the recording sheet in order to fix the toner image to the recording sheet.
Patent Literature 1 (Japanese Patent Application Publication No. 2000-227732) discloses a fixing device that controls the surface temperature of a heat roller heated by a heating means, such as a halogen heater, to be a predetermined temperature by detecting the surface temperature using an infrared sensor. In this fixing device, the infrared sensor is moveable parallel to the axis of the heat roller, so that one infrared sensor can detect the surface temperature at a plurality of positions along the axis of the heat roller. Based on variation in the measured surface temperature, the heating means, such as a halogen heater, is controlled.
In recent years, a system has also been adopted wherein a resistance heating element that generates heat by conduction is used as the heating means in a fixing device. In this system, the resistance heating element is, for example, provided in a rotating heating belt. The outer circumferential surface of the heating belt and a pressing roller press against each other to form a fixing nip, and recording sheets pass through the fixing nip.
The resistance heating element provided in the heating belt is supplied with power at either edge in the direction of width (along the rotation axis), which is perpendicular to the direction of rotation of the heating belt. The resistance heating layer produces Joule heat due to the current flowing along the direction of width. The heat thus produced in the resistance heating layer traverses the fixing nip and is applied to the recording sheet. The toner image on the recording sheet is thus thermally fixed.
In this sort of fixing device, since the heating belt, which is the source of heat, has a low heat capacity, the warm-up time can be kept short. Moreover, since the distance from the resistance heating layer in the heating belt to the recording sheet is short, heat produced in the resistance heating layer is efficiently applied to the recording sheet. Accordingly, the amount of consumed energy can be reduced both during warm-up and during fixing operations.
In a fixing device that uses a heating belt with a resistance heating layer, however, the problem occurs that the resistance heating layer may be damaged by improper jam clearance when a jam occurs or by a foreign object attached to the recording sheet. If the damage to the resistance heating layer, such as a scratch, occurs along the circumferential direction of the heating belt (a direction perpendicular to the direction in which current flows in the resistance heating layer, i.e. perpendicular to the direction of width of the heating belt), a locally high temperature may be reached along the circumference of the scratched location.
The reason for occurrence of a high temperature is as follows. If a scratch occurs along the circumferential direction of the resistance heating layer, then along the circumference of the scratch, current cannot flow in the direction of width of the heating belt. Rather, the current has to flow around the scratch. As a result, current becomes locally concentrated in the circumferential direction of the resistance heating layer at either circumferential end of the scratch. The current density thus increases at the circumferential ends of the scratch. As a result, the circumferential ends of the scratch overheat, reaching a locally high temperature.
Such a locally high temperature in the heating belt may cause image noise, such as high temperature offset. Furthermore, if a long scratch occurs in the circumferential direction of the heating belt, the current density rises even more at either circumferential end of the scratch, which may lead to an abnormally high temperature. In this case, the fixing device may suffer damage, such as melting of the surface of the pressing roller that presses against the heating belt.
Therefore, it is preferable to detect that damage, such as a scratch, has occurred on the resistance heating layer of the heating belt in order to prevent problems such as image noise and damage to the pressing roller.
As described above, in the case of a scratch in the circumferential direction of the resistance heating layer, the temperature at the circumferential ends of the scratch becomes high. Therefore, if portions with a locally high temperature are detected, it can be determined that a scratch has occurred in the resistance heating layer.
For example, the infrared sensor disclosed in Patent Literature 1 detects an average temperature within a measurement region, defined as a constant range on the surface of the opposing heating belt (a range over a fixed area at one location in the direction of width of the heating belt). While such an infrared sensor displaces the measurement region along the entire circumferential surface of the rotating heating belt, if the average value of the measured temperature obtained at a predetermined sampling time is higher than a preset threshold temperature, it can be determined that a scratch has occurred in the resistance heating layer within the measurement region.
In this case, however, since the infrared sensor detects an average temperature for a measurement region with a constant area, the average temperature along the entire circumferential surface of the heating belt might not be equal to or greater than the predetermined threshold temperature even if the temperature at the circumferential ends of a scratch on the resistance heating layer is at least the threshold temperature. This is because the measurement area of the infrared sensor at the sampling time may be larger than the locally high temperature portions, causing the measured temperature (average temperature) in the measurement area at the sampling time to be lower than the actual temperature of the locally high temperature portions. In such a case, even though the resistance heating layer has been scratched, the scratch cannot be detected.
Furthermore, even when it can be detected that a scratch to the resistance heating layer has occurred, the length in the circumferential direction of the scratch is unclear. As a result, use of the image forming apparatus may be restricted due to suspension of fixing operations, even when the scratch in the resistance heating layer is not long in the circumferential direction to pose the risk of damage to the pressing roller.